1. Field of the Invention
The present invention relates generally to a method, user equipment, and system for opening an ad-hoc Push-To-Talk (PTT) over Cellular (PoC) session in a PoC system, and more particularly to a method, user equipment, and system that can open an ad-hoc PoC session for a call target group whose group Identifier (ID) is managed in a remote network domain when the ad-hoc PoC session is opened.
2. Description of the Related Art
With the significant development of mobile communication and extension of communication networks, various extended services and applications using mobile phones are being provided. Due to user demands, services beyond the basic call service such as positioning, multimedia and Push-To-Talk (PTT) services, are being provided. Specifically, the PTT service supports various additional functions such as instant messenger and state display, as well as a group call and a voice call that are possible in a conventional radio system or a Trunked Radio System (TRS).
A concept of this PTT service is being actively discussed to standardize a PTT over Cellular (PoC) service using a mobile communication network. One characteristic that distinguishes the PoC service from the conventional mobile communication service is that users can perform communication while moving between multiple sessions as needed. The requirement that users should be able to perform communication while moving between multiple sessions is prescribed in the standard of the Open Mobile Alliance (OMA) serving as an organization for developing a standard for a mobile communication service.
This OMA PoC technology exploits a Session Initiation Protocol (SIP)/SIP extension protocol corresponding to a typical application layer protocol for controlling Internet Protocol (IP) multimedia telephony in order to deliver session participation information of a group call and exploits an Extensible Markup Language (XML) Configuration Access Protocol (XCAP) for managing group information.
A PoC spec currently being standardized by the OMA includes a PoC service that supports a multiparty conference function of a half-duplex communication scheme and supports various types according to purpose of a group. That is, PoC conferences are classified into ad-hoc, prearranged and chat PoC groups according to characteristics of a participating group.
An ad-hoc group call and a prearranged group call open a dial-out session by a client sending a session open request to a server acting as a focus (hereinafter conference server) and the conference server sends a session request to respective clients belonging to a group.
A chat group call has characteristics of a session open and join procedure in which clients have dial-in access to a conference server of a chat group when they recognize in advance associated session identification information or group session information provided through group advertising.
To open a PoC multimedia session, a PoC call requester at a transmitting side makes a request for processing a call by sending a multimedia INVITE message using the SIP. At this time, the multimedia INVITE message can include various formats of audio, video, text, and so on according to media types. A receiving side performs various answer procedures according to presence of a prearranged session and answer mode setting.
To process a PoC call, a PoC Client A at the transmitting side sends an SIP INVITE message containing SIP address information of a target receiver to an associated SIP/IP core network A. The SIP INVITE message contains elements of PoC address information of the client sending a call request, required media parameters, and characteristic value information indicating the PoC service. The SIP INVITE message is delivered to a Participating PoC server (or Participating PoC Function (PF) entity) through an associated IP Multimedia Subsystem (IMS) server (of a Proxy-Call Session Control Function (P-CSCF) and Serving-Call Session Control Function (S-CSCF)) using a path query in a Dynamic Host Configuration Protocol (DHCP) server or Domain Name Server (DNS). When a multimedia session is requested, the required media parameters include a plurality of characteristic values indicative of parameters such as an audio and video encoding method, rate and payload type.
When a basic call request is made, the PF entity connected to a PoC user can be implemented which is separated from the controlling PoC server (or Controlling PoC Function (CF) entity) for managing a talk burst of an opened session. The SIP INVITE message sent to the PF entity is transferred to the CF entity through the associated SIP/IP core network.
A PoC session control network with the CF receives a response message from a network at the receiving side after transferring an SIP INVITE message to the network at the receiving side. The SIP response message from the network at the receiving side can be a provisional response message of 1XX, a successful response message of 2XX, or an error response message of 4XX˜6XX according to setting of the PF and the PoC client at the receiving side. In the case of the automatic answer mode, an SIP 183 “session progress” signal can be received, through which a connection between the PoC server and client can be established in the IMS network of a call requester. A call acknowledgment signal of the PoC client at the receiving side is returned as an SIP 183 “session progress” signal or an SIP 200 “OK” response, and is transferred to the PoC client A through the PoC server of the CF and PF.
After receiving the SIP 200 “OK” response or the SIP 183 “session progress” signal from the PoC server at the receiving side, the CF entity determines that a PoC call has been connected and sends a floor granted signal for assigning the floor for a talk burst to the PoC client A. The response of the SIP 200 “OK” or 183 “session progress” signal can be classified as a “confirmed” or “unconfirmed” response. The CF entity requires a buffering function when the “unconfirmed” response is received.
The PoC client A at the transmitting side receives a talk burst transmission granted signal, for example, a floor granted signal for transferring call connection sound, through a Real Time Control Protocol (RTCP) after receiving the response signal to the SIP INVITE message. The floor granted signal is generated from the CF entity having the right to arbitrate a talk burst and is sent to the PoC client through the PF entity for managing an associated PoC client. Since the floor granted signal uses a bearer path without the SIP, it can be sent without passing through the SIP/IP core network such as the IMS. When identifying the call connection sound, the PoC user sends a media stream (or a typical voice stream) using a Real-time Transport Protocol (RTP).
As defined in the PoC 1.0 standard document, an ad-hoc PoC group session can be opened according to edition of the PoC client at the transmitting side. At this time, target PoC address information mapped to an ad-hoc group is constructed with a single SIP or TEL Uniform Resource Identifier (URI). According to function for opening a flexible ad-hoc group session newly defined in the PoC 2.0 standard, a call target group can have a group URI rather than a single URI.
For this, the PoC client at the transmitting side can open the flexible ad-hoc group session by including a group type indicator for the group URI in a message to be sent to open the ad-hoc PoC group session and performing a session procedure. In this case, when a call target group URI is managed in a domain equal to that of a PoC server operating as the CF entity in an ad-hoc session, an ad-hoc group session management server can open the ad-hoc group session by receiving information of clients mapped to an associated group identifier from a PoC XML Document Management Server (XDMS).
However, when the call target group URI is not managed in a domain (hereinafter a home network domain) equal to that of the PoC server operating as the CF entity in the ad-hoc session, the ad-hoc group session management server cannot receive client information mapped to an associated group from a PoC XDMS within a different domain. That is, the ad-hoc group session management server should send a session invitation to associated clients by directly collecting a list of clients belonging to a remote network other than the home network. Conventionally, a method for efficiently opening a flexible ad-hoc group session is not provided for when a call target group identifier used to open an ad-hoc session is not managed in a PoC server within a home network domain or a connected PoC XDMS.